An intraocular lens (IOL) is a surgically implanted, polymeric lens designed to replace the natural crystalline lens in the human eve, typically in patients who have developed visually significant cataracts. Since their inception in the late 1940's, IOLs have provided improved uncorrected visual acuity (UCVA) compared to that of the cataractous or aphakic state; however, problems in predictably achieving emmetropia persist as most post-cataract surgery patients rely on spectacles or contact lenses for optimal distance vision. Compounding the issues related to achieving optimum distance vision, patients undergoing cataract surgery lose their ability to accommodate, i.e. the ability to see objects at both near and distance.
The determination of IOL power required for a particular post-operative refraction is dependent on the axial length of the eve, the optical power of the cornea, and the predicted location of the IOL within the eye. Accurate calculation of IOL power is difficult because the determination of axial length, corneal curvature, and the predicted position of the IOL in the eye is inherently inaccurate. (Narvaez el al., 2006; Olsen, 1992; Preussner et al., 2004; Murphy et al., 2002). Surgically induced cylinder and variable lens position following implantation will create refractive errors, even if preoperative measurements were completely accurate. (Olsen, 1992) Currently, the options for IOL patients with less than optimal uncorrected vision consist of post-operative correction with spectacles, contact lenses or refractive surgical procedures. Because IOL exchange procedures carry significant risk, secondary surgery to remove the IOL and replace the first IOL with a different power IOL is generally limited to severe post-operative refractive errors.
With current methods of IOL power determination, the vast majority of patients achieve a UCVA of 20/40 or better. A much smaller percentage achieves optimal vision without spectacle correction. Nearly all patients are within two diopters (D) of emmetropia.
In a study of 1,676 patients, 1,569 (93.6%) patients were within two diopters of the intended retractive outcome. (Murphy el al., 2002). In 1,320 cataract extractions on patients without ocular co-morbidity, Murphy and co-workers found that 858 (65%) had uncorrected visual acuity greater than 20/40. (Murphy et al., 2002). A 2007 survey of cataract surgeons reported that incorrect IOL power remains a primary indication for foldable IOL explanation or exchange. (Mamalis et al., 2008; and Jin et al., 2007)
In addition to imprecise IOL power determinations, post-operative uncorrected visual acuity is most often limited by pre-existing astigmatism, Staar Surgical (Monrovia, Calif.) and Alcon Laboratories (Ft. Worth, Tex.) both market a toric IOL that corrects pre-existing astigmatic errors. These IOLs are available in only two to three tone powers (2.0, 3.5 D and 1.50, 2.25 and 3.0 D, respectively at the IOL plane) and the axis must be precisely aligned at surgery. Other than surgical repositioning, there is no option to adjust the IOL's axis which may shift post-operatively. (Sun et al., 2000) Furthermore; individualized correction of astigmatism is limited by the unavailability of multiple toric powers.
An additional problem associated with using pre-implantation corneal astigmatic errors to gauge the required axis and power of a toric IOL is the unpredictable effect of surgical wound healing on the final refractive error. After the refractive effect of the cataract wound stabilizes, there is often a shift in both magnitude and axis of astigmatism which off-sets the corrective effect of a toric IOL. Therefore, a means to postoperatively adjust (correct) astigmatic refractive errors after lens implantation and surgical wound healing is very desirable. While limbal relaxing incision is a widely accepted technique for treating corneal astigmatism, the procedure is typically performed during cataract surgery; therefore, the procedure does not address the effect of post-implantation wound healing.
In the United States alone, approximately one million eyes undergo corneal refractive procedures which subsequently develop cataracts, thus, presenting a challenge with respect to IOL power determination. Corneal topographic alterations induced by refractive surgery reduce the accuracy of keratometric measurements, often leading to significant post-operative ametropia. (Feiz et al., 2005; Wang et al., 2004; Latkany et al., 2005, Mackool et al., 2006; Packer et al., 2004; Fam and Lim, 2008, Chokshi et al., 2007, Camellin and Calossi, 2006). Recent studies of patients who have had corneal refractive surgery (photorefractive keratectomy, laser in situ keratomileusis, radial keratotomy) and subsequently required cataract surgery frequently demonstrate refractive “surprises” post operatively. As the refractive surgery population ages and develops cataracts, appropriate selection of IOL power for these patients has become an increasingly challenging clinical problem. The ability to address this problem with an adjustable IOL is valuable to patients seeking optimal distance vision after cataract surgery.
Accommodation, as it relates to the human visual system, refers to the ability of a person to use their unassisted ocular structure to view objects at both near (e.g. reading) and far (e.g. driving) distances. The mechanism whereby humans accommodate is by contraction and relaxation of the ciliary body, which connects onto the capsular bag surrounding the natural lens. Under the application of ciliary stress, the human lens will undergo a shape change effectively altering the radius of curvature of the lens. (Ciuffreda, 1998). This action produces a concomitant change in the power of the lens. However, as people grow older the ability for their eyes to accommodate reduces dramatically. This condition is known as presbyopia and currently affects more than 90 million people in the United States. The most widely accepted theory to explain the loss of accommodation was put forth by Helmholtz. According to Helmholtz, as the patient ages, the crystalline lens of the human eye becomes progressively stiffer prohibiting deformation under the applied action of the ciliary body. (Helmholtz, 1969). People who can see objects at a distance without the need for spectacle correction, but have lost the ability to see objects up close are usually prescribed a pair of reading glasses or magnifiers. For those patients who have required previous spectacle correction due to preexisting defocus and/or astigmatism, they are prescribed a pair of bifocals, trifocals, variable, or progressive focus leases which allows the person to have both near and distance vision. Compounding this condition is the risk of cataract development as the patient ages.
To effectively treat both presbyopia and cataracts, the patient can be implanted with a multifocal IOL. The two most widely adopted multifocal IOLs currently sold in the United States are the ReZoom® (Abbott Medical Optics, Santa Ana, Calif.) and ReStor® (Alcon, Fort Worth, Tex.) lenses. The ReZoom® lens is comprised of five concentric, aspheric refractive zones, (U.S. Pat. No. 5,225,858). Each zone is a multifocal element and thus pupil size should play little or no role in determining final image quality. However, the pupil size must be greater than 2.5 mm to be able to experience the multifocal, effect. Image contrast is sacrificed at the near and for distances, to achieve the intermediate and has an associated loss equivalent to one line of visual acuity, (Steiner et al., 1999). The ReStor® lenses, both the 3.0 and 4.0 versions, provide simultaneous near and distance vision by a series of concentric, apodized diffractive rings in the central, three millimeter diameter of the lenses. The mechanism of diffractive optics should minimize the problems associated with variable pupil sizes and small amounts of dec-entration. The acceptance and implantation of both of these lenses has been limited by the difficulty experienced with glares, rings, halos monocular diplopia, and the contraindication for patients with an astigmatism of greater than or equal to 2.0 D. (Hansen et al., 1990; and, Ellingson, 1990). Again precise, preoperative measurements and accurate IOL power calculations are critical, to the success of the retractive outcome, and neither the ReZoom nor the ReStor lenses provide an opportunity for secondary power adjustment post implantation. (Packer et al., 2002).
One of the newest concepts proposed to tackle the dual problems of cataract's and presbyopia are through the use of accommodating IOLs. Two companies, Bausch & Lomb (Rochester, N.Y.) and Human Optics AG (Erlangen, Germany) have developed IOLs that attempt to take advantage of the existing accommodative apparatus of the eye in post implantation patients to treat presbyopia. Bausch & Lomb's lens offers a plate haptic configured IOL with a flexible hinged optic (CrystaLens®). Human Optics's lens (AKKOMMODATIVE® ICU) is similar in design, but possesses four hinged haptics attached to the edge of the optic. The accommodative effect of these lenses is caused by the vaulting of the plate IOL by the contraction of the ciliary body. This vaulting may be a response of the ciliary body contraction directly or caused by the associated anterior displacement of the vitreous body. Initial reports of the efficacy of these two lenses in clinical trials was quite high with dynamic wavefront measurement data showing as much as 2 D to 3 D (measured at the exit pupil of the eye) of accommodation. However, the FDA Ophthalmic Devices' panel review of Bausch & Lomb's clinical results concluded that only a 1 D accommodative response (at the spectacle plane) was significantly achieved by their lens, which is nearly identical to the pseudo-accommodation values achieved for simple monofocal IOLs.
A need exists for an intraocular lens which is adjusted post operatively in-vivo to form a presbyopia correcting intraocular lens. This type of lens can be designed in-vivo to correct to an initial emmetropic state (light from infinity forming a perfect focus on the retina) and then the presbyopia correction is added during a second treatment. Such a lens would (1) remove the guess work involved in presurgical power selection, (2) overcome the wound healing response inherent to IOL implantation, and (3) allow the amount of near vision to be customized to correspond to the patient's requirements. Also, an intraocular lens which is adjusted post operatively in-vivo to form an aspheric optical element would result in the patient having an increased depth of focus (DOF), which allows the patient to see both distance and near (e.g. 40 cm) through the same lens.